The present invention relates to a relief valve. More particularly, the present invention relates to a thermally activated relief valve for use with a compressed gas storage cylinder (or pressure vessel).
It is desirable for some compressed gas storage cylinders to have safety valves such as a thermal relief device (TRD). For example, cylinders which hold compressed natural gas are required by law to have such relief valves.
In the past, such cylinders have used thermal relief valves which are activated when the body of the thermal relief valve exceeds a temperature threshold. For example, thermal relief valves of the prior art would typically have a passage which communicated with the interior of the cylinder. However, the passage would be filled (blocked) with a eutectic metal slug which remained solid below the predetermined melting temperature of the eutectic. Then, if the temperature of eutectic substance raised above the predetermined melting temperature, the slug would soften or melt. In its softened or weakened state, the eutectic substance would be forced out of the passage in the relief valve by the pressure of the gas in the cylinder. With the eutectic substance removed from the passage, the gas would be released through the passage to the external atmosphere or another suitable container.
However, such a relief valve was plagued by certain problems. The problems primarily resulted because the eutectic metal was exposed to the gas pressure in the cylinder. Under such pressure, the eutectic substance underwent what is known as plastic flow or creep. In other words, the gas pressure acting on the eutectic block over time would push some of the eutectic block out of the passage. Thus, even though no over-temperature condition existed, the thermal relief valve would be partially activated and leak. Since the eutectic substance, exposed to the gas pressure, was prone to plastic flow or creep, the life expectancy of such a thermal relief valve was relatively short. Thus, such relief valves need to be replaced more often than desirable.
Combination pressure relief and thermal relief devices have typically used a eutectic metal "slug" backed by a thin metal disc. Although this design solves the eutectic creep problem, it obviously requires a minimum gas pressure to rupture the disc, and has thus proven to be inoperative on partially filled cylinders. A letter warning against the use of this type of relief device was circulated by the United States and Canadian Gas Associations several years ago.
Since the eutectic creep problem is diminished with the use of smaller and/or partially restricted bores/openings, one solution is to simply limit the physical size of the TRD. While this is a perfectly practical solution for small volume cylinders, it is totally impractical for cylinders used on large trucks or buses. A recent CNG Urban Bus demonstration project utilizing three (3) long storage cylinders mounted on the bus roof required a total of twenty-seven (27) TRDs to meet the required emergency flow rate.
The advance of high pressure composite technology has made it feasible and economically attractive to expand the size and pressure capabilities of storage cylinders for compressed natural gas used in vehicular applications. These new design, light weight, composite cylinders are typically constructed using a thin wall metal or plastic liner which is over-wrapped with multiple layers of fiberglass/epoxy resin or graphite fiber/epoxy resin. In either case, the majority of the cylinders' burst strength is provided by the fiberglass or graphite/resin over-wrap. Because the over-wrap material is more susceptible to damage in a fire than metal, this cylinder design is more vulnerable to fire than conventional all-metal storage cylinders.
To adequately protect these "space age" cylinders from wrap strength degradation in a fire, the cylinders must be protected/fitted with high flow, fast acting, thermal relief safety devices. It should be noted that older style thermal devices (either open throat or convoluted/mazed) do not function (activate) consistently in these applications because the relatively cold stored gas inside the "insulated" cylinder cools the flow blocking eutectic mix as the gas begins to exit. This causes the eutectic to "re-freeze" and block the gas exit path. In this scenario, the fire causes the outer wall temperature of the cylinder to continue to escalate, while the eutectic safety device goes through a succession of freeze/thaw (melt) cycles during which time (typically 8-12 minutes) very limited amounts of compressed gas are released. This situation leaves the cylinder very vulnerable to catastrophic failure because of the cylinders' reduced burst strength.
As disclosed in U.S. Pat. No. 5,161,738, a relief valve utilizing a hollow bayonet to puncture a metal disc seal is known in the art. The bayonet design disclosed therein functions effectively to pierce the metal disc when it has a thickness no greater than 0.005 inch. At high operating pressures, the portion of the metal disc that is cut by the prior bayonet design is completely exhausted ("digested") out of the relief valve. At operating pressures of about 500 p.s.i. or less, or when the metal disc has a thickness greater than 0.005 inch, the prior bayonet design has been found to be incapable of consistently and dependably "digesting" the metal disc. As such, the portion of the metal disc that is cut by the prior bayonet design has a tendency to become lodged in the hollow bayonet, thereby preventing an unobstructed flow of gas through the relief valve.
At a thickness of 0.005 inch, the metal disc forming the seal has an over-pressure burst value of approximately 5400 p.s.i. Recent rule changes in filling procedures for storage cylinders used in compressed natural gas-operated vehicles allow 125% over-filling of such storage cylinders. Thus, storage cylinders having a working pressure of 3600 p.s.i. are now permitted to be filled to 4500 p.s.i. (125%.times.3600 p.s.i.). In light of this recent change, it has become necessary to use thicker metal disc seals to avoid incursion into the upper 30% of the disc pressure range (where cycle fatigue is likely to occur). A metal disc having a thickness of about 0.007 inch has a burst value of about 7500 p.s.i. The ability to use a metal disc seal having a thickness of 0.007 inch in a hollow bayonet-type relief valve would provide about a 2000 p.s.i. margin over the previous industry guideline limit of 150% of working pressure (which is 5400 p.s.i. for a 3600 p.s.i. system). This in turn would provide considerable stability and reliability to the relief valve.
There is, therefore, a need for a relief valve which meets or exceeds all of the critical performance criteria necessary for the newer cylinder designs, as well as for the filling procedures for CNG automobile storage cylinders. Such a relief valve must satisfy the following conditions:
(1) Fast Action--Typical fire activation time of 2-3 minutes is needed to allow compressed gas to begin venting before the over-wrapped cylinder's burst strength is significantly reduced. PA1 (2) Very High Reliability--The design must have the following features to assure high reliability: PA1 (3) Relatively High Flow--The relief valve must open a relatively large exhaust port to achieve high flow (exhaust) rates.
(a) The relatively cool exiting gas stream can not resolidify the eutectic to possibly cause a catastrophic failure due to delayed activation. PA2 (b) Cylinder gas pressure can not exert an "extruding" force on the eutectic when the eutectic becomes susceptible to "plastic flow" (creep) as the system temperature approaches/encroaches on the eutectic melt temperature (during routine operation). PA2 (c) The bayonet design must be capable of consistently and dependably rupturing the metal disc seal to expose a flow path for the contents of the storage cylinder.